In the manufacture of glass and other transparent substrates, it is often necessary to inspect the body of the transparent solid material for defects. In particular, detecting small inclusions that lie within the volume of the glass, where the inclusions are of micron or even submicron order, presents a considerable challenge. Inclusions within a glass medium can generally be classified in two groups: solid inclusions, which are formed by bits of unmelted or foreign material; and void inclusions, commonly formed by bubbles of gas. Solid inclusions can be formed by minute impurities in the starting materials which have been fused to form a glass; by bits of refractory material from the walls of the vessel in which the glass is prepared; or by impurities that are otherwise introduced during glass manufacture. The solid inclusions may be opaque or clear. Void inclusions, or gas bubbles, also present difficulties in visual inspection.
Inclusions can be particularly troublesome for high-purity glass materials such as those that serve as substrates for microlithography exposure masks and photomasks, for example. Often fabricated using processes other than flow process techniques, such as by vapor deposition, glass materials of this type can be very expensive to produce. Costly and time-consuming finishing processes are often needed in order to provide a final product from a glass boule manufactured in this way.
An inclusion inspection system that is suitable for high-value glass substrates of this type must meet the following performance criteria:                (i) Sufficient speed. This requirement is of increasing importance as the relative size and volume of the glass substrate increases.        (ii) Able to adapt to substrate thickness over a range.        (iii) Good location specificity. The more accurately the location of an inclusion can be specified the better. When a defect location can be pinpointed, the affected area of the substrate can be removed from surrounding portions that pass inspection.        (iv) Good sensitivity to inclusions. An inspection system for high-quality glass media should be able to detect micron- and sub-micron-level inclusions.        (v) Insensitivity to surface quality. It is beneficial to detect inclusions within the volume of the glass as early as possible in the surface finishing cycle, before the surface is finely polished or otherwise treated to obtain an optical finish.        
Where some of these requirements may work against each other, a reasonable balance must be achieved. For example, with reference to the criteria noted earlier, maximizing sensitivity (iv) could compromise speed (i) and robustness to surface quality (v). Conversely, maximizing speed (i) could have an adverse effect on both sensitivity (iv) and location specificity (iii).
Thus far, conventional inspection solutions for glass inclusions may meet one or two of these performance requirements, but fail to meet all five of these criteria. For example, manual inspection methods have been used and continue to be used for inclusion detection in a number of specialty glass manufacture environments. In order to use these methods, the glass surface is first polished to an optical finish. Then, high-intensity light wands are employed to painstakingly examine the inner volume of the glass. This method has a number of shortcomings. Speed (i) is a significant drawback. Another drawback is the requirement for a finished surface, as noted in criterion (v). There are limits on light types and intensities that are available, in consideration of operator safety. Further problems relate to high cost and relatively low repeatability.
Optical microscopy has also been used for detecting inclusions. Microscopy is characterized by an extremely narrow depth of field and a small sampling area at high magnification. While this solution is optimized for sensitivity, its slow speed (i) can be a significant drawback, making microscopy impractical for inspection of large volumes of bulk glass material.
Automated methods developed thus far for inclusion detection also have operational and performance drawbacks. Among proposed approaches for automated inclusion detection are techniques that employ Total Internal Reflection (TIR). TIR techniques use waveguide properties of the material under inspection. In practice, TIR methods are suitable only where the glass medium is relatively thin. These methods also require a highly finished surface and cannot be used effectively with thicker substrates, failing to meet the performance criteria identified earlier under items (ii), (iii) and (v). One illustrative example of this conventional method using TIR for a thin moving web of glass is described in U.S. Pat. No. 4,401,893 entitled “Method and Apparatus for Optically Inspecting a Moving Web of Glass” to Dehuysser. Another example for inspecting aircraft glass is given in U.S. Pat. No. 5,517,301 entitled “Apparatus for Characterizing an Optic” to Dave.
Side-lighting is another conventional method that has been described for glass inspection. This method is hampered by unevenness in the illumination path and other problems such as the requirement for a finished surface, requirements for high-intensity sources, and some inherent diffraction at glass edges and scattering. A flat, polished edge is required for the incident light. With larger sized glass sheets, the bowing of longer sections can cause additional problems. As a result, this method is constrained to bodies of glass having limited length and width dimensions. Thus, side illumination techniques fail to meet inspection criteria (ii), (iv), and (v) listed earlier and are impractical for glass boules and where lengths of glass exceed about 20 inches. One example of a side-lighting technique is described in U.S. Pat. No. 3,737,665 entitled “Method and Apparatus for Automatically Detecting Defects and Irregularities in Glass Sheet” to Nagae.
Another approach has been to direct light into the substrate and to block all light detectable by a sensor except for the scattered light caused by inclusions. This type of approach is described, for example, in commonly assigned U.S. Pat. No. 6,388,745 entitled “Detecting Inclusions in Transparent Sheets” to Stevens et al. and in U.S. Pat. No. 6,633,377 entitled “Dark View Inspection System for Transparent Media” to Weiss et al. These approaches can detect inclusions above a certain size, but are not capable of providing accurate information on inclusion depth, thus fail to meet criteria (ii), (iii), (iv), and (v).
Yet another approach has been to direct a curtain of HeNe laser light into the glass surface, with one or more cameras poised at oblique angles for detecting scattered laser light from inclusions, as described in U.S. Pat. No. 5,459,330 entitled “Process and Device for the Inspection of Glass” to Venaille et al. This type of approach is not well-suited to rough surfaces and, because it is prone to generating secondary scattering from the top surface, fails to meet the requirements given earlier as criterion (ii), making it difficult to isolate true inclusions from surface defects. It also fails to meet sensitivity requirements (iv). Where there is surface curvature, the resulting refraction can make it difficult to completely scan the full volume of a glass substrate without gaps. Where both top and bottom surfaces have curvature, multiple reflections can occur within the field of view of sensing optics. In summary, methods such as those described in the Venaille et al. '330 disclosure fail to meet criteria (i), (ii), (iv), and (v) for inclusion inspection systems for high-value glass.
All of the techniques noted earlier suffer from the same significant limitation: failing to meet criterion (v). That is, each of these conventional techniques requires a highly finished, optical quality surface. This means that the glass must be fully cycled through the manufacturing process before it can be tested for inclusions. It can be appreciated that this entails additional expense and represents wasted effort in some cases, since an inclusion can render the glass sheet, or some portion of the glass medium, useless for its intended application and this defect cannot be found until a significant amount of value-added manufacturing has been applied to the glass substrate.
As noted earlier, other related shortcomings of conventional approaches include disappointing performance due to problems such as light loss, intensity variation, and dimensional limitations, particularly acute with methods using TIR and side-lighting methods, such as those cited. Conventional approaches have been developed and used for glass having surfaces that are in a relatively finished state, such as glass fabricated by flow processes, but do not satisfy the more demanding requirements of inspection for high-quality glass formed using deposition or other alternative processes for which a sequence of finishing and polishing procedures may be needed in order to properly condition the surface.
With respect to performance criteria (i) through (v) listed earlier, conventional approaches have proved to be deficient in at least one or more of these requirements. Thus, it can be appreciated that there is a need for a glass inspection method and apparatus that allows accurate inspection of the bulk or internal volume of a glass substrate medium that provides sufficient speed, that does not require a highly finished surface and is not constrained to thin sheets of glass, that identifies the location of inclusions at specific locations, and that exhibits sufficient sensitivity for detecting very small inclusions.